My colleague wrote her own urban fantasy novel, and while it is a goldmine of bad history (stuff like Vikings in 5th century Britain in a backstory of one of vampire characters), the most egregious example is one of characters claiming that "Dark Ages" had more suffering than World War 2, and that people born after year 1800 don't know what true suffering is.

I could try to make bad history thread, in theory, but I will refrain since I don't feel in mood for posting in detail about someone who is not a stranger to me.

PS: She's rewriting and editing her novel right now, and I and my friends convinced her to correct some of more blatant ahistorical stuff like Vikings.